Although rotary internal combustion engines have reached a degree of commercial acceptance, considerable interest is now being devoted to improving fuel economy and durability of such engines. The water cooling system for such an engine is particularly relevant to attaining these two goals. The housing water cooling system, in a rotary engine, functions to lower the temperature of the metal areas exposed to the highest heat input and to minimize temperature differences throughout the housing for preventing destruction. The most severe cooling problem resides in the area where combustion and expansion of the working gases takes place; this area immediately surrounds the spark plugs. The uneven heating can cause housing distortion which, in turn, can prevent proper functioning of the gas and oil sealing elements. The time during which the combustion chamber is cooled by fresh inducted air is fairly short allowing the wall temperature of the combustion chamber to be high and sensitive to changes in load. The maximum temperature of the combustion surface of the trochoid wall is much higher than that of the housing side walls; local overheating can destroy the oil film on the trochoid surface. Sudden acceleration with a cold engine, especially in winter or when auto ignition occurs during high speed driving, exposes the rotor housing and associated trochoid wall to repeated sudden and very large thermal loads. As a result, thermal fatigue or thermal shock cracks can appear about the spark plug holes. In general, cracks occur most frequently on the gas side of the trochoid wall and along the spark plug holes in the axial direction in conformity with high stress concentrations. In extreme cases, cracks can even reach the water jacket. There is a greater need for perfection in design to limit this tendency for thermal distortion which is so highly dependent on the relationship between the cooling system, housing and rotor seals.
One particular design aspect that has assumed commercial acceptance, is the use of in-line or dual spark plugs for a single rotor housing. The reason for the dual in-line spark plugs is as follows: In a rotary piston engine with a rotor rotating eccentrically along an inside surface having a trochoid curve, it is ideal for the spark plugs to be installed on the trochoid surface close to the minor axis of the curve, from the standpoint of engine output. However, since the compressed air-fuel mixture also undergoes a rotating motion along with the rotation of the rotor, the rotary engine has a characteristic flame front which advances to the leading side of the rotor and has very little propagation to the trailing side of the rotor. Therefore, the air-fuel mixture disposed in the trailing portion of the rotor combustion pocket is not completely burned. Consequently, the exhaust gas will contain a large amount of unburned gaseous components. To remedy this, another or auxiliary spark plug is installed downstream from the first spark plug and the latter is moved slightly upstream; the auxiliary spark plug is ignited after the first spark plug has been ignited, or in certain cases they may be ignited simultaneously. The necessity for the in-line arrangement is due to the physics of propagation and the desire to have the entire air-fuel mixture totally combusted. The optimum location to do this was thought to be in the center of the peripheral wall whereby the flame front would advance in the direction of movement of the air/fuel mass and proceed laterally across the shortest path toward each of the side walls to combust all of the mixture. Unfortunately, the in-line arrangement of such spark plugs creates a mechanism by which the flow of cooling fluid is considerably disrupted, vapor films collect, and the flow is prevented from carrying away the heat in such a critical area.
Spark plugs for an internal combustion engine, such as a rotary, are typically installed into the threaded ports of the spark plug bosses. Since a rotary engine has a relatively thin trochoid wall, cylindrically shaped bosses for the spark plugs must be cast and extend into the engines water jacket passageway which is adjacent to such wall. The interruption or interference of such bosses within the water jacket passageway has a benefit in that the bosses themselves are cooled to carry away heat but the total heat for the entire hot spot area is detrimentally affected; the cooling flow is extremely sensitive to hindrances preventing heat extraction. Each boss in a four-cylinder reciprocating engine will be affected by generally 1/4 of the total heat of combustion for the engine. This is not a severe problem in connection with reciprocating type internal combustion engines since the spark plug bosses are well separated in the cylinder heatt water jacket and, in fact, can be considered as one spark plug per cylinder. However, in contradistinction, the spark plug bosses in a rotary engine are cast in close proximity to the circumference of each rotor housing, do not have special coolant transfer ports for improved cooling, and are generally affected by 1/2 of the total heat of combustion for a two rotor engine (for a one rotor engine, the bosses would be effective by the total undivided heat of combustion).
As the rotary design has developed, spark plugs have been fitted into the threaded ports which open onto the most critically cooled zone of the trochoid combustion surface - a major hot spot where thermally induced structural failures are more likely to occur. If the cooling flow cannot carry away the heat in a uniform manner, the exact amount of excess heat in such hot spot will cause detrimental results. The in-line arrangement of spark plug bosses in such water passageway contributes, in a significant manner, to preventing adequate heat extraction. Particularly in the vertically upward flow of the cooling circuit, where in-line spark plugs are typically placed, the up-stream plug boss creates a flow shadow effect upon the down-stream plug boss preventing a controlled or well ordered flow regime (absence of swirling eddies which deteriorate heat transfer). Boiling at the plugs results in a vapor stream which widens the uncontrolled flow zone and aggravates the heat transfer problem.